The present invention relates to reduced noble metal catalysts. More particularly, the present invention relates to reduced noble metal catalysts on shaped metal oxide supports having high loadings and dispersions of the noble metal.
Supported metal catalysts may be conveniently described by reference to the total metal content thereof measured as percent of dry catalyst weight and to the dispersion of the metal on the catalyst surface. Metal dispersion is a measure of effective availability of metal and is inversely related to metal crystallite size. Larger crystallite sizes (smaller metal dispersion values) result in a loss of metal availability due to occlusion of metal atoms within the crystallites.
It has been previously known to prepare noble metal catalysts on shaped metal oxide supports such as extrudates. However, such catalysts either possess low metal loadings, e.g., less than about 0.45 millimoles of noble metal/gram catalyst, or if higher loadings were obtained, the metal dispersion was relatively low, e.g., less than about 50 percent. Catalysts having low metal dispersions are undesirable since for all practical purposes, noble metal that is unavailable for catalytic activity is wasted. At the same time, catalysts having high metal dispersions obtained only at low metal loadings are equally wasteful due to the additional bulk of the catalyst support. Not only are larger volume reactors required in order to accommodate the extra volume of inert support thereby adding to process costs, but additional pressure losses attributable to the increase in catalyst volume may render a process uneconomical due to the increased operating pressures required. It would be desirable to prepare a supported noble metal catalyst having high metal loadings and dispersions. In particular, it would be desirable to prepare a supported noble metal catalyst having metal loadings greater than 0.45 mmoles/g of dry catalyst and having a dispersion greater than 50 percent.
Values of metal dispersion may be easily calculated for any metal and substrate if an average crystallite size is known. For example, metal dispersion (in percent) for a palladium catalyst is defined as 1122/D where D is the mean crystallite size measured in angstroms. The above formula may be applied to palladium catalysts regardless of the type, shape or size of the catalyst support, or the manner of preparation of the catalyst. Similar formulas for determining metal dispersions for other noble metal catalysts are contained in Anderson, Structure of Metallic Catalysts, pp. 360-63, Academic Press, NY (1975).
Numerous techniques are known for the preparation of noble metal catalysts. Examples include acid solution impregnation, ion-exchange, base precipitation, etc. In J. R. Anderson, Structure of Metallic Catalysts, Academic Press (1975), p. 199, it was stated that on silica supports the best palladium dispersion was obtained by cation-exchange. Average particle diameters of 14 angstroms at a 2.2 weight percent palladium loading (0.2 mmole/g) were reported. The largest loading and dispersion on alumina reported by the reference were those obtained by P. C. Aben, J. Cat., 10, 224-229 (1968). The author alleged preparing palladium on alumina having 5 percent (0.47 mmole/g) and 15 percent (1.42 mmole/g) metal loadings and about 51 percent and 35 percent dispersions, respectively. At page 228, Table 4, note C, the author provided a formula for converting experimental hydrogen chemisorption ratios to crystallite size (in angstroms). The formula provided was D=100.times.0.885/H/Pd. A more accepted formula for calculating dispersion is D=100.times.H/Pd. Generalizing this equation to other metals one obtains: EQU D=100.times.(W/M);
wherein D equals percent dispersion and W/M is the atomic ratio of chemisorbed hydrogen to the noble metal. Techniques for measuring free metal surface areas are disclosed generally in Scholten, J. J. F., "Preparation of Catalyst II," from Studies in Surface Science and Catalysis, Vol. III; ed. B. Delmon, P. Grame, P. Jacobs, G. Poncelet; Elsevier, Inc., New York, New York, 1979 and Scholten, J. J. F., Surface Characterization of Supported and NonSupported Hydrogenation Catalysts, Catal. Rev.--Sci. Eng., 27(1), 151-206 (1985), both of which are incorporated herein by reference. The corresponding values of H/Pd provided by the reference and the resulting calculated values of D and percent dispersion are:
______________________________________ Particle Size % Sample H/Pd (angstroms) Dispersion* ______________________________________ 5% w Pd/Al.sub.2 O.sub.3 0.41 22 41 15% w Pd/Al.sub.2 O.sub.3 0.28 32 28 ______________________________________ *Calculated from the formula 100 .times. H/Pd
Using techniques of carbon monoxide absorption, J. J. F. Scholten et al. Journal of Catalysis, 1, 85-92 (1962) alleged that a commercial catalyst with 5.5 percent (0.52 mmole/g) palladium on .gamma.-Al.sub.2 O.sub.3 had measured values of mean crystallite size of from 13.7 angstroms to 15.3 angstroms, depending on the measuring process employed. These values would correspond to a metal dispersion of either 82 percent or 73 percent. However in Scholten, (1979), supra., doubt is cast on these results. At pages 696 and 697, Scholten states that the carbon monoxide method has the difficulty that the chemisorption stoichiometry is variable because the proportion of chemisorbed species in the linear and bridged forms can vary. The carbon monoxide adsorbed depends on pressure temperature and metal particle size.
It has been previously known to prepare supported metal catalysts employing powdered rhodium supports having large loadings and dispersions. For example, H. C. Yao et al. disclosed powdered alumina catalysts having 5.3 percent (0.51 mmole/g) metal and 57 percent dispersion in J. Catal., 50, 407 (1977). Powdered catalyst supports are particularly ineffective in some commercial practice due to the large pressure drop associated with a reactor filled with a powdered catalyst.
In numerous commercial processes such as hydrogenation of unsaturated compounds, e.g., edible oils, and carbonylation reactions, large amounts of catalyst are employed in reactors of large volumes and bed length. In such operations, powdered catalysts, e.g., noble metal deposited on powdered metal oxide substrates, compact and fail to provide sufficient pore volume to allow ready permeation of the gaseous or liquid reactants which must pass through the catalyst bed. The consequent large pressure drop across the catalyst bed results in extremely limited throughput and correspondingly excessively high working pressures. Accordingly, it is highly desirable to prepare a noble metal catalyst on an extruded or shaped metal oxide support having sufficient size and porosity to provide permeation of gaseous or even liquid reactants therethrough without the use of excessive pressure differentials.
Equally necessary for certain processes, e.g., carbonylation reactions, high conversions and catalytic effectiveness are desired in order that the process be economically operated. Production costs and market price are directly related to capital costs associated with the process. An extremely costly item involves the catalyst and the necessary reactors, and associated handling, loading, pumping and contacting equipment. As a particular illustrative example, it has been found that a vapor phase process for the preparation of methyl methacrylate by reaction of 2-chloropropene, methanol and carbon monoxide under conditions of elevated temperature and pressure is rendered economically viable only by the use of a noble metal catalyst having high metal loadings and high metal dispersions.